Leonsevero Passeri

An overview of the maldivian coral reefs in Felidu and North Malé Atoll (Indian Ocean): Platform drowning by ecological crises

SummaryThis paper gives general information on: (1) sedimentary environments and carbonate producers in some Maldivian atolls (North Malé Atoll and Felidu Atoll); (2) theAcanthaster planci outbreaks in North Malé Atoll and (3) the geological evolution of the Maldives.(1)The distribution and the main features of the reefs are described. The rim of the atolls can be erosional or bioconstructed; the seaward side is often erosional while coral reefs thrive in the lagoonward side and in the lagoonal patch reefs. Attention is paid to the distribution of the main hard coral genera in the different environments of the atolls (oceanward rim, lagoonward rim, passes, lagoonal patch reefs and faro lagoons). GenusDistichopora (Idrozoa) andZoopilus (Scleractinia) are reported in the Maldives for the first time; many sclearactinian species not prerviously reported are illustrated. Evidence of the importance ofTydemania (Codiacea) as a Maldivian producer of fine-grained carbonates is provided.(2)General inforomation aboutA. planci and its outbreaks in the Maldivian atolls is given. The spawning of the MaldivianA. planci occurs in the spring during the monsoon inversion withChaetodon falcula (a butterfly fish) being an important predator ofA. planci eggs. The skeleton of the Maldivian variety ofA. planci is briefly described.Plerogyra, Physogyra andEuphyllia are reported for the first time among the scleractinians surviving after theA. planci invasion.(3)DSDP and ODP data were used to calculate the subsidence rate of the Maldivian atolls (from 70 to 30 m/my); this value is much lower than the presumed upward growth rate of a thriving reef (7–8 mm/yr). Only the glacio-eustatic sea-level rise (similar to the post-Wurmian rise) has attained this value. These observations suggest that the drowning of an atoll or of a shallow water carbonate platform may mainly be due to ecological crises that impair the carbonate production of the framebuilding communities. Crises can arise from changes in climate or in oceanic circulation, from emersion followed by sudden sea-level rise or from the invasion of predators.

HETTANGIAN AMMONITES AND RADIOLARIANS IN THE MT. CAMICIA (GRAN SASSO, CENTRAL APENNINES)

The Vallone di Vradda stratigraphic section is situated in the eastern part of the Gran Sasso range and shows the transition from Late Triassic euxinic facies to Early Liassic open pelagic facies. This paper describes Middle and Late Hettangian ammonite and radiolarian assemblages found in the upper part of the succession. The existence of an anoxic event below the Middle Hettangian beds is noted. Assemblages of small-sized Middle Hettangian ammonites suggest some taxonomic innovation within the early Lytoceratina: gen. n. (Pleuroacanthitidae) and Analytoceras n. sp. indet. Radiolarians found together with ammonites improve the knowledge of Hettangian radiolarian assemblages of the western Tethys.

Facies and Late Triassic fossils in the Roisan zone, Austroalpine Dent Blanche and Mt Mary-Cervino nappe system, NW Alps

The Roisan zone is a metamorphic cover unit exposed along the ductile shear zone between the Dent Blanche s.s. and Mont Mary-Cervino Upper Austroalpine outliers, Aosta Valley, north-western Italian Alps. It is characterized by the occurrence of dolostones, pure marbles, marbles with quartz, calcirudites and ophiolite-free calcschists. Locally, dolostones preserve alternances of thick massive beds and thinner levels of planar stromatolites and other sedimentary structures and textures typical of a carbonate platform. In Mt Grand Pays they contain Dasycladales and foraminifers referable to the Norian. Pure marbles and marbles with quartz grains are tentatively referred to the end of Triassic–Early Jurassic, thin-bedded marbles and calcirudites to the Early and Middle Jurassic, calcschists from Middle Jurassic to Late Cretaceous. This Roisan succession is quite similar to the one of Mt Dolin, in the Swiss part of the Dent Blanche nappe, where the same Triassic foraminifer association has been reported. There, the fossils were found only in reworked pebbles, contained in calcirudites of presumed Jurassic age. Some differences exist between the two successions: calcirudites are abundant in the Mt Dolin and sporadic in the Roisan zone, whereas calcschists are very thick in the Roisan zone. As consequence the Mt Dolin succession can be considered settled down in the proximity of the faults related to the pre-oceanic rifting of the Piedmont basin, whereas the Roisan zone could have been deposited in a more distal area.

The significance of Longobucco Unit (Calabria-Peloritani Arc) in the evolution of the Ionian and Alpine Oceans

In the geological literature the Longobucco Unit, as well as the whole Calabria-Peloritani Arc (CPA), was interpreted in opposite ways: 1) as part of the African-Adria Plate (thus belonging to Austroalpine Units) previously involved in the eo-Alpine deformation with an european vergence and then thrust southeastwards on the Apennine chain; 2) as parts of the European continental margin involved only in the Apenninic orogeny; 3) as part of a microplate between two branches of the Alpine ocean (Pennidic O. and Ligurian O). In this paper we contribute to the solution of this controversial problem with basin analysis on the Longobucco Mesozoic cover, taking into account the paleogeographic evolution of the West Mediterranean area during the Mesozoic.1) At first, the sedimentary successions of the Longobucco Unit (Early Jurassic-Miocene) are re-examined. The peculiar occurrence of conglomerates with pebbles of Variscan basement from the Early Jurassic, the existence of carbonate ramps attached to an exposed continental area in the Hettangian-Sinemurian and the huge amount of silici-clastic grains in the entire Mesozoic succession witness for the deposition on a continental margin exposed to either erosion and synsedimentary tectonics.2) The coeval successions of the Apennines (Adria Plate), southern Sicily (African Plate) and eastern Sardinia (European Plate) are compared to understand which exposed areas had to be the source for the terrigenous sediments. Our conclusion supports the hypothesis (already formulated by various authors) that from the oceanic opening of the Alpine Ocean (Middle Jurassic) the Longobucco Unit and the whole CPA were part of the European Plate.3) The comparison between the nappes stack of CPA and that well known in the Western Alps confirms the idea that the CPA cannot be considered as Austroalpine Unit. In fact, despite the same occurrence of granulitic facies, the differences are remarkable: different dip of subduction plane, different vergence, different age of HP/LT metamorphism, different age of exhumation and even different Mesozoic cover. Therefore CPA has to be considered as part of the European Plate.4) The original position of CPA is very important to define the relationship between the Ionian and the Alpine Oceans in the western Mediterranean area. Here, the oceanic spreading of Ionian basin occurred in the Ladinian and, from the Late Eocene, this ocean was subducted northwards below the European and Adria Plates.Oceanic spreading of the Alpine trough began in the Middle Jurassic and, from the Late Cretaceous, this ocean was subducted south and eastwards below the Adria Plate. The European origin of the CPA indicates a possible existence of direct communication between those two oceans. In our interpretation this connection was related to a sinistral trasform fault that crossed the western part of the modern Mediterranean area from the Late Triassic.5) The presumed existence of a Jurassic microcontinent (AlKaPeCa) is critically discussed and it appears inconsistent with stratigraphic data. In conclusion, CPA (and the whole AlKaPeCa) cannot be considered as Austroalpine Units or a microcontinent, but as a marginal part the European plate involved in the north-dipping subduction of the Ionian Ocean. This subduction produced a southvergent stacking of the various Calabrian units and began around 43 Ma below the CPA, long before the involvement of the Corsica continental margin in the east-dipping subduction of the Alpine Ocean (34 Ma).

The problematic origin of the Pancherot-Cime Bianche-Bettaforca unit (PCB) in the Piemonte zone (Western Alps)

The Pancherot-Cime Bianche-Bettaforca (PCB) is an ophiolite-free decollement cover unit with a greenschist facies imprint, continental affinity and problematic provenance. It is located in the ophiolitic Piemonte zone which marks the suture in the north-western Alps of the Mesozoic Liguria-Piemonte ocean (western Tethys). A new stratigraphic reconstruction of this unit is presented. It includes the following main lithofacies: Quartzitic Schists and metaconglomerates (Permian), Yellowish Quartzite (Early Triassic), Bedded Dolostone and Marbles (Middle Triassic), Tabular White Quartzite (Carnian), Massive Dolostone (Norian), Carnieules and Brown Marble (Rhaetian-Jurassic). Ages are hypothetical because no fossil has been found.Comparison with the Prepiemontese and Austroalpine sedimentary successions shows major affinity with the Austroalpine cover units.The continental basement of this exotic unit is unknown. Suitable candidates can be considered the Mt Emilius and other Lower Austroalpine eclogitic outliers, as these continental slices are missing of a sedimentary cover and are located inside the ophiolitic Piemonte zone, near the structural level of the PCB unit. In this view, thesebasement and cover units were decoupled during subduction due to their contrasting metamorphic features.